The following U.S. patents are believed to represent the current state of the art: U.S. Pat. Nos. 6,263,113 B1; 6,108,437; 6,035,074; 6,006,231; 5,978,507; 5,963,670; 5,915,038; 5,905,807; 5,896,462; 5,793,888; 5,764,790.